1. Technical Field
The present invention relates to power assisted rack and pinion steering systems for vehicles. More specifically, the present invention relates to the connection between a pinion and a valve sleeve, and the connection between a torsion bar and an input shaft, in a fluid power assisted rack and pinion steering gear assembly.
2. Description of the Prior Art
In a typical fluid power assisted steering system for a vehicle, a rotary valve directs the flow of hydraulic fluid under pressure to a hydraulic motor. The motor moves steering linkage to effect turning of the steerable wheels of the vehicle. The rotary valve includes a valve core and a valve sleeve which are relatively rotatable. A steering input shaft extending from the vehicle steering hand wheel has a portion formed as the valve core. One end of a torsion bar is rotationally fixed to the input shaft. An output member is rotationally fixed to the valve sleeve. The other end of the torsion bar is rotationally fixed to the output member.
When there is a resistive load on the output member from the steerable vehicle wheels, steering torque transmitted through the input shaft causes the torsion bar to twist. This action actuates the valve from a neutral condition and changes the rotary position of the valve core relative to the valve sleeve, in turn changing the fluid flow through the valve and to the motor. The motor moves the steering linkage to turn the steerable vehicle wheels. Steering linkage movement moves the output member in a follow-up manner to rotate the valve sleeve relative to the valve core and return the rotary valve to its neutral condition.
In a fluid power assisted rack and pinion steering gear assembly, the pinion is the output member. One end of the torsion bar is press fitted to the pinion. The other end of the torsion bar is pinned to the input shaft by a drilling, reaming and pinning operation. One such rack and pinion steering gear is shown in U.S. Pat. No. 4,996,905. In such a steering gear assembly, the input shaft and the pinion are supported for rotation at only three places along their length. These parts unavoidably undergo small bending movements because of forces applied to the steering gear assembly.
One location where such movement is manifested is the location where the valve sleeve is connected for rotation with the pinion. Typically, a cylindrical hitch pin is press fitted in the pinion and extends through a cylindrical opening in the valve sleeve. The opening in the valve sleeve is slightly larger than the diameter of the hitch pin, because of manufacturing tolerances. This size difference allows for radial and axial movement of the hitch pin in the opening resulting from the above-mentioned bending of the parts. This size difference, however, undesirably allows some relative angular movement between the pinion and the valve sleeve.
Another location where such bending movement is manifested is the location where the inner end of the torsion bar rotates within the input shaft. Typically, the adjoining surfaces of these parts are closely fitted to limit relative radial movement of the parts while still allowing relative rotational movement. For example, in U.S. Pat. No. 4,996,905, a bearing surface on an inner end portion of the torsion bar supports a portion of the input shaft for rotation relative to the torsion bar. Machining the parts to close tolerances as needed to operate in this manner is difficult and expensive.